This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-072196, filed Mar. 15, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a method of decomposing an ammonia gas.
Ammonia generated in the gasification process of coal acts as a poison when the product gas is utilized in the presence of a catalyst, and acts as an NOx source when the product gas is combusted in a gas turbine. Therefore, removal of ammonia from the product gas is an essential technique for the gasification process of coal.
In general, ammonia is removed from the product gas by decomposing with the aid of a catalyst. As such a catalyst, disposable materials such as dolomite containing iron, sintered iron ore or limestone have been studied. However, such a material has not made it possible to decompose sufficiently the ammonia gas yet.
Such being the situation, it is proposed to use a VIII Group transition metal based catalyst such as Ni or Ru supported by an alumina carrier or the like. The catalyst of this type permits decomposing ammonia with a relatively high efficiency.
However, the VIII Group transition metal based catalyst is costly and, thus, is unsuitable for use in a process of decomposing a large amount of an ammonia gas.
As described above, it was impossible in the past to decompose an ammonia gas with a low cost and with a sufficiently high efficiency.
An object of the present invention is to provide a method of decomposing an ammonia gas with both a low cost and a sufficiently high efficiency.
According to a first aspect of the present invention, there is provided a method of decomposing an ammonia gas into a nitrogen gas with use of a composite material as a catalyst, the catalyst comprising a carrier consisting essentially of carbon and at least one active element supported by the carrier and selected from the group consisting of alkaline earth metals and transition metals.
According to a second aspect of the present invention, there is provided a method of decomposing an ammonia gas into a nitrogen gas with use of a catalyst, the catalyst being at least one of a composite material obtained by heating an organic material containing at least one active element selected from the group consisting of alkaline earth metals and transition metals so as to thermally decompose the organic material, and a composite material obtained by heating a mixture of at least one active element selected from the group consisting of alkaline earth metals and transition metals and an organic material so as to thermally decompose the organic material.
In the present invention, a composite material comprising a carrier consisting essentially of carbon and at least one active element supported by the carrier and selected from alkaline earth metals and transition metals is used as a catalyst for decomposing an ammonia gas. Where the particular composite material is used as a catalyst, it is possible to decompose an ammonia gas into a nitrogen gas with a very high efficiency. In addition, the particular composite material can be manufactured from materials available easily with a low cost. It follows that the present invention permits decomposing an ammonia gas with a low cost and with a sufficiently high efficiency.
In the present invention, it is desirable for the ammonia gas to be decomposed at the temperature of 500 to 1,200xc2x0 C., preferably 700 to 900xc2x0 C. Where the decomposition temperature of the ammonia gas is not lower than 500xc2x0 C., the ammonia gas can be decomposed at a very high degree of decomposition. Also, if the temperature exceeds 1,200xc2x0 C., a considerably large portion of the ammonia gas is decomposed even if a catalyst is not present. Under the circumstances, the effect of the present invention is rendered more prominent, if the decomposition temperature of the ammonia gas is set at a level not higher than 1,000xc2x0 C.
It is desirable to decompose the ammonia gas under the pressure of 0.1 to 10 MPa, preferably 0.1 to 2 MPa. Where the ammonia gas is decomposed under the pressure noted above, it is possible to realize a very high degree of decomposition of the ammonia gas.
The composite material used as a catalyst in the present invention can be obtained, for example, by thermally decomposing an organic material containing at least one active element selected from the group consisting of alkaline earth metals and transition metals. Alternatively, the composite material used as a catalyst in the present invention can be obtained by heating a mixture of at least one active element selected from the group consisting of an alkaline earth metals and a transition metals and an organic material so as to thermally decompose the organic material. These methods are simple and the raw materials noted above are very cheap.
The organic material is not particularly limited as far as the organic material produces a carbonaceous material when the organic material is thermally decomposed. For example, the organic material used in the present invention includes coal such as brown coal, subbituminous coal, bituminous coal or anthracite coal, peat coal, algal coal, heavy oil, woody waste such as biomass, waste wood, lumber from thinning, or sawdust, and a plastics waste. Particularly, it is desirable to manufacture the composite material by using a material having a high oxygen content such as brown coal, subbituminous coal, peat coal or algal coal. In the case of using such a material, it is possible to decrease the diameter of the particles formed of the elements noted above to a nanometer order.
The active element supported by a carrier in the present invention is not particularly limited as far as at least one element selected from the group consisting of alkaline earth metals and transition metals is used as an active element supported by the carrier, though it is desirable for iron and/or calcium to be supported by the carrier because of low coast. Where the composite material contains the active element noted above, the ammonia gas can be decomposed with a very high efficiency. The ammonia gas can also be decomposed with a high efficiency in the case where the composite material contains an alkaline earth metal such as magnesium, strontium, or barium or a transition metal such as chromium, manganese, nickel, cobalt, copper, zirconium, molybdenum, palladium, tungsten, rhenium, osmium, iridium, silver, ruthenium, rhodium, gold or platinum.
Where the composite material used as a catalyst in the present invention contains as the active element only one of the alkaline earth metal and the transition metal, it is desirable for the concentration of the active element in the composite material to fall within a range between 0.1 and 10 mass %, more preferably between 1 and 3 mass %. Where the composite material contains as the active elements both the alkaline earth metal and the transition metal, it is desirable for the concentration of the active elements in the composite material to fall within a range between 0.1 and 20 mass %, more preferably between 1 and 5 mass %.
In the present invention, the particle diameter of the composite material is not particularly limited. To be more specific, it is possible for the composite material to be in the form of particles having a diameter of several centimeters or in the form of a powder having a particle diameter of several hundred microns. Further, it is possible to mold the powdery composite material into pellets.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.